Will Increases in Processing Power Outpace PenTile RGBW LCD Savings in Display Power?

Recently I read with interest a string on PenTile technology at Howard Forums Mobile Community http://bit.ly/rDSNiJ where several good points were made and insightful questions were asked.  In today’s blog I will try to address a question that was raised here that jasaero rightfully said we have never addressed.

In the Howard Forums blog  rkeller62 making the point that as resolution continues to climb any pattern visibility becomes less apparent, much like the halftone in a magazine.  He also mentioned visual adaption, a process where the human vision system filters artifacts that do not do not contain data.  I had pointed out in a prior blog http://pentileblog.com/?p=879 that this adaption has happened with the jail bars of RGB stripe and is already happening for many with the checkerboard pattern of PenTile technology.

To this jasaero raised the valid question of whether or not we really needed all of that extra resolution that would be needed to avoid pattern visibility.  He pointed out that these extra pixels add to computational power that could well outweigh the power savings of PenTile RGBW LCDs. He argues that such computation power would have to increase in proportion to the increase in the number of pixels.

Let me begin with the question of whether or not we need all of those additional pixels.  My reply is that it depends upon how you use your smartphone.  Judging from the various blog entries on this topic, there are many who seem to value more resolution, so I can tell that these people would appreciate formats with more pixels.  The question is, at what point would resolution exceed the capability of the human vision system to see such patterns?

For this, I have the chart below showing resolution in PPI as well as cycles/degree of human vision.  The human vision system (HVS) doesn’t care about PPI, but rather sees cycles/degree—how many black and white line pairs can be fit into one degree of view entering your eyes.  Displays at a given PPI will have various levels of cycles/degree depending upon how closely one is accustomed to viewing this display.  In my chart, I make some assumptions about viewing distance by application, but we know from reading blogs that there are some who can accommodate (focus) on a display from extraordinarily close range.  If these people have good visual acuity at this range they will see things like pattern visibility that were not intended by the designers of this produce.  That is true for RGB stripe as well as PenTile technology.

If you have somewhat aging eyes it may well be that the 3.65”, 316 ppi, Nexus One when viewed at 30 cm would be beyond the limits of your resolution, but for normal vision we would need to go to 50 cy/degree to exceed useful resolution.  For those with visual acuity of 50 cy/degree, PenTile pattern visibility will totally disappear for the new Nexus Prime when it is viewed at 32.5 cm.  For those with 50 cy/deg of acuity who prefer to view it still closer, we could use still a bit more resolution.

So let’s consider the question of increased power consumption for supporting all of those extra pixels.  Without debate, there is more power required to support more pixels, but the power of the AP (including the GPU) consumes at least two orders of magnitude less power than that required by the display .   Display backlights are one of the larger consumers of power in smartphones, only surpassed by the radio.  The power used by 4G LTE is considerable.  Dual core high power processors are using more power, but still this is orders of magnitude less power than the display backlight.

Why is this?  It is because TFT LCDs are horribly inefficient.  Even at modest resolution like TVs, LCDs only transmit about 10% of the light from the backlight to your eye.  In higher resolution displays one is doing well to get 5% of the light out the front.  Where does all of this light go?  Half is wasted by the light absorbtion of the polarizers, more is absorbed by the opaque structures in the backplane electronics array, more is absorbed in the color filter array, still more is lost though back reflection at various interfaces of the many layers in the display. Saving half the power in the display backlight is much more important than doubling the power of the AP that consumes orders of magnitude less power.  As displays grow in diagonal, if the brightness is maintained, the power will increase as the square of diagonal!

Even so, we are always trying to reduce the processing power for PenTile technology, as small as this currently has become.  Eliminating one-third of the gate drivers helps, but this is offset by gates needed to perform our adaptive filtering and our dynamic backlight control.  Overall the power consumed by logic that is only used by PenTile is roughly the same as the power saved by eliminating drivers.  At some point, however, PenTile logic will be placed into the AP.  At that point it will be possible to drop the data rate by one-third which will substantially decrease logic power and at the same time reduce EMI associated with the display.  In the meanwhile, Android users have benefited from having a tool to monitor power consumption to see exactly what is the culprit in their phone.

There is currently, and for the near term, no other technology that saves as much display power as PenTile technology.  If you enjoy high resolution PenTile technology continues to be a good companion technology.

 

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